JSFiddle - React, Tailwind, and code Playground

HTML

<h1>JS - Curiosidade</h1>
<form name="calc">
  <input type="text" name="visor" /><br/>
  <input type="button" value="7" onclick="botao( value );" />
  <input type="button" value="8" onclick="botao( value );" />
  <input type="button" value="9" onclick="botao( value );" />
  <br/>
  <input type="button" value="4" onclick="botao( value );" />
  <input type="button" value="5" onclick="botao( value );" />
  <input type="button" value="6" onclick="botao( value );" />
  <br/>
  <input type="button" value="1" onclick="botao( value );" />
  <input type="button" value="2" onclick="botao( value );" />
  <input type="button" value="3" onclick="botao( value );" />
  <br/>
  <input type="button" value="0" onclick="botao( value );" />

  <input type="button" value="Duplicar" onclick="botao( visor.value );" />
  <br/>
</form>

JavaScript

botao = function(num) {
  var salvo = document.calc.visor.value;
  document.calc.visor.value = salvo + num;
}